1. Field of the Invention
This invention relates to a high voltage power cable assembly having reduced stray capacitance.
2. Discussion of the Background
The high voltage power cable assembly is used for connection of a high voltage transformer with an X-ray tube in an X-ray diagnostic apparatus. The high voltage transformer provides 60 to 150 kV rectangular pulses to the X-ray tube through the high voltage cable assembly. However, the high voltage power cable assembly has stray capacitance which causes the rise and fall times of the rectangular pulses to be delayed. Especially since the delayed fall time of the rectangular pulses results in increased pulse width, it impedes the high speed scanning of successive pulses.
A conventional high voltage power cable 3 as shown in FIG. 1 is used for the connection of a high voltage transformer with an X-ray tube under regulation of JIS (Japan Industrial Standard) C3407. Such a cable 3 includes two low voltage conducting lines 11A, 11B and high voltage conducting line 14 in the center thereof. The two low voltage conducting lines 11A, 11B are led to a cathode of the X-ray tube and are respectively connected to filament coils for large and small focus spots of the X-ray tube. The high voltage conducting line 14 is also led to the cathode of the X-ray tube and is connected to a common tap of the filament coils. The conducting lines 12A, 12B and 14 include many stranded or twisted wires forming conductors 10A, 10B and 9. The surface of conductors 10A, 10B are covered with an insulating conduit 11A, 11B, such as EP (Ethylene Propylene) rubber. The surface of conductor is covered with a semi-conductive conduit 13, such as semi-conductive EP rubber. This semi-conductive conduit 13 has a smooth surface to increase the insulating voltage rating, and performs in a manner similar to a Faraday shield. JIS permits the omission of this semi-conductive conduit 13. These conducting lines 12A, 12B and 14 are stranded or twisted along the center axis of the cable 3.
The twisted conducting lines 12A, 12B and 14 are covered with a semi-conductive tube 15 to form a cable core 16. This semi-conductive tube 15 increases the insulating voltage rating, in a way similar to Faraday shields.
The cable core 16 is covered with a high voltage insulating layer 17, such as EP rubber. The surface of the high voltage insulating layer 17 is covered with a shield layer 18 including wires such as copper or tin-gilt copper interwoven with fibers such as cotton fibers. The surface of shield layer 18 is covered with a sheath 19, made of, e.g., chloroprene or a vinyl. Each dimension is shown in following Table 1.
TABLE 1 ______________________________________ Conductors High voltage Low voltage ______________________________________ Structure (lines/mm) 19/0.32 19/0.32 Diameter (mm) 1.6 1.6 Thickness of semi- 0.8 -- conductive rubber (mm) Thickness of -- 0.8 insulating rubber (mm) Thickness of semi- 0.8 conductive tube (mm) Diameter of core (mm) 8.5 Thickness of high 4.0 voltage insulating layer (mm) Thickness of 0.3 shielding layer (mm) Thickness of sheath (mm) 1.2 Total diameter (mm) 19.5 ______________________________________
The stray capacitance Cx of the high voltage power cable 3 is represented by the following equation: ##EQU1## where .epsilon. is the dielectric constant of the insulating layer 17, D is the diameter of the insulating layer 17 and d is a diameter of the cable core 16.
Equation (1) indicates that if the ratio D/d increases, the stray capacitance Cx decrease. However, it is not desirable to increase the diameter D, because it becomes less flexible in a computed tomography apparatus which repeatedly winds or rewinds the cable.